Gallium-nitride (GaN) devices are known to lack ruggedness to withstand overvoltage stress, especially avalanche stress. The ability of silicon devices such as MOSFETs to withstand voltage spikes with reasonable energy levels has been widely used as an extra level of protection in the design of power electronic circuits. Users of such devices have come to expect that transistors can handle voltage spikes, and it significantly reduces the complexity of power electronic design if they can. It can also allow the use of lower breakdown voltage rated devices, since the user can reduce the amount of voltage guard band applied to protect against voltage surges. Such surges can arise from grid voltage anomalies, lightning strikes, current transients interacting with inductive circuit elements, and other causes.
Current GaN devices address this weakness by overdesigning the transistors to break at a voltage far in excess of the rated voltage. For example, a device with a nominal 650 volt maximum drain-source voltage rating might be designed with a breakdown voltage of 800-1100 volts, while a silicon device with a rating of 650 volts will usually have a breakdown voltage of 700-750 volts. In some applications, this overdesign causes the GaN device to be larger and higher in on-resistance, increasing the cost and reducing the performance in the intended application. Further, in some applications it has also limited the use of GaN devices, since users expect their power transistors to have overvoltage clamping capability, and are reluctant to change to a device that might compromise the ruggedness or reliability of their circuit.
To better understand the nature and advantages of the present invention, reference should be made to the following description and the accompanying figures. It is to be understood, however, that each of the figures is provided for the purpose of illustration only and is not intended as a definition of the limits of the scope of the present invention. Also, as a general rule, and unless it is evident to the contrary from the description, where elements in different figures use identical reference numbers, the elements are generally either identical or at least similar in function or purpose.